ISDN signaling is defined by the ISDN standard Q.931 and is intended to provide an international standard to control the initiation of calls, progress of calls, termination of calls, communication of national use information, local serving network information, and user-specific information for telecommunications systems and terminals. The ISDN standard defines two standard links: (1) primary rate interface (PRI) links, each having 23 data channels (B channels) and one signaling channel (D channel), and (2) basic rate interface (BRI) links, each having two B channels and one D channel. The ISDN message format allows each message to have a number of information elements, IEs, and groups IEs by codesets. There are eight possible codesets, numbered 0-7. Codeset 0 is the currently defined set of information elements and includes IEs that control the initialization, maintenance, and termination of calls. Codesets 1-4 are reserved for future ISDN standard Q.931 expansion. Codeset 5 is for national use; codeset 6 is for IEs specific to the local serving network; and codeset 7 is for user-specific IEs.
In a system having a plurality of switching nodes, a characteristic of ISDN signaling is that each intervening switching node in the signaling path (which comprises a number of standard links) must process messages communicating the signaling information since each intervening switching node terminates the signaling channel. This characteristic creates three major problems.
First, each intervening switching node processes and interprets all IEs (codeset 0) relating to control of initiation, progress, and termination of calls. The result is that terminal equipment cannot be transparently remoted (not directly connected) from a controlling switching node through intervening switching nodes using standard links if standard signaling is used for call control. Nor can two switching nodes transparently exchange control information if there are intervening switching nodes. Terminal equipment can only be remoted and switching nodes can only directly exchange control information if non-standard signaling is used for call control or dedicated links are used. However, non-standard signaling requires that both switching nodes and controlled terminal equipment function differently than if the controlled terminal equipment was directly connected to the controlling switching node or the two switching nodes were directly connected to each other.
The second problem is that if standard signaling is used, the transmission rate of the signaling information is greatly reduced because of the need to process that information in each intervening switching node.
And finally, a large additional processing load is placed on the intervening switching nodes.
As was previously described, the ISDN standard intends for terminal equipment to be directly controlled via the signaling channel by the local switching node to which the terminal equipment is attached since any intervening switching node processes and interprets all IEs performing call control. Hence, although the ISDN standard is adequate for most types of telecommunications applications, this standard suffers from serious problems when it is desirable to remote terminal equipment from a controlling switching node through another switching node such as a public network. In order to implement such remote applications, the remote terminal equipment must function in a different manner if standard ISDN call control signaling is not used, or dedicated links must be used if ISDN call control signaling is used.
The most disadvantageous result of using standard signaling without dedicated links is that the remote terminal equipment cannot be utilized in the same manner as terminal equipment directly connected to the controlling switching node. There are however many applications where it is desirable, in fact, to allow remote equipment to have the same capabilities that it would have if it were directly connected to the controlling switching node. One such example is where the controlling switching node is a PBX, and it is desirable to control a small number of telephones at a remote location. Because of the distance involved, it is often desirable to connect this terminal equipment to the PBX via the public network. Office personnel operating procedures make it desirable for the users of the remote terminal equipment to be provided the same functions as the users of terminal equipment directly connected to the PBX. For a small number of remote users, it is not economical to put a switching system at the remote location capable of terminating a PRI link and switching the channels from this link to the users. Another application is the need in the telemarketing business to be able to place a remote telemarketing terminal in a private home and to connect that terminal to a controlling switching node via the public network and a BRI link.